In theory, the Internet Protocol version 4 (IPv4) is capable of uniquely addressing over 4 billion networked devices simultaneously. However, inefficient address allocation schemes of the past, as well as the rapid, worldwide proliferation of Internet-enabled devices, have led to the near-exhaustion of IPv4 addresses. IP version 6 (IPv6) has been proposed as a replacement for IPv4, and provides a much larger address space. However, in order to upgrade a network to IPv6, most of the network's client devices, routers, gateways, and application servers would need to support or interoperate with IPv6. Even if such an upgrade is technically feasible, the cost of the upgrade, including the cost of training network administrators to understand and properly manage IPv6 devices, could be significant. Therefore, it is likely that IPv4 will continue to be used in the near future.